Asymmetries/dysbalances in the preferred vs. the non-preferred kicking leg of football players

Having a preferred kicking and jumping leg it would seem obvious to have a stronger and a potential weaker leg.

From a technical and tactical perspective it seems interesting to have dominantly left footed players (like Robben), playing on the right and vice versa (like Ribery). However, are there any
consequences with regards to a health (injury/injury prevention) or performance perspective?

The most common measurements however utilized an isokinetic dynamometer (8, 13, 17) to test hip flexor (12), knee flexor (5, 8, 14, 17) and extensor (5, 8, 12, 14), hip (14), trunk muscles in
concentric (14) and/or eccentric (14) motions with different speeds such as 60, 120, 240 degrees/second (5). Comparisons are made between muscle groups of the opposite leg (preferred vs.
non-preferred leg) (12, 14) in different movement velocity. As a result there are multiple combinations of the mentioned variable that have been used throughout the literature.

It was shown that the preferred side was weaker than the non-preferred leg (14) in knee extension and flexion muscle strength, however, no differences were found for hip flexibility in English
Premier league professional (14).

Interestingly, poor performers in the FMS showed bilateral differences, while the good performers did not in German professional footballers (16). In addition, it seems that training experiences
(years played football) showed an affect on asymmetries, with more experienced players showed less dysbalance between the preferred vs. the non-preferred kicking leg in professional footballers
from Greece (8).

There were also no dysbalances in various testing velocity muscle contraction types and/or muscle groups in another study utilizing similar participants (professional football players of the
first Greek division) (17).

Asymmetries/dysbalances and its connection to injury

It was concluded that lopsided training lead to muscular imbalances which is generally regarded as an injury risk factor (14). In addition, limb length differences was reported as an intrinsic
risk factors in football players (7), showing that asymmetries/dysbalances increased the hamstring strain injury risk 3.8-fold.

Is a link prevalent in youth football players?

It seems that there is a few scientific research with regards to imbalances in youth/young footballers which reveals conflicting results.

Chin et al. (4) did not observe any isokinetic strength differences for the knee extensors however in the knee flexors between dominant and non-dominant limbs in elite Asian junior football
players.

Kellis et al. (10) tested talented Greek football players aged 10-17 and found significant strength imbalances for knee flexors and extensors between dominant and non-dominant limbs. From the
data I calculated a maximum of asymmetries of 11% between the dominant vs. non-dominant leg.

Capranica et al. (2) did not report any limb asymmetries in 10-year old footballers.

What is the threshold for having an athlete “at risk”?

Kirkendall (11) stated in 1985 that football players need to stay within the clinical norms (10%) in contralateral muscle imbalance to avoid sport injuries. Dauty et al. (6) stated that
concentric hamstring torque asymmetry of more than 10% identified the injured soccer players.

However, Fowler et al. (10) stated that 20% differences in bilateral muscle strength in professional footballers were seen as a risk factor.

Croisier et al. (5) reported a bilateral strength differences for hamstring of 15% or more, constitutes an athletes at risk.

Conclusion

From the literature it seems very “hard” to give a defined answer about a direct link of preferred to non-preferred kicking leg asymmetries/dysbalances to injuries. From one perspective it seems
that limb asymmetries are a “normal” phenomenon in footballers.

However, as limb length and bilateral muscle strength were seen as intrinsic risk factors, it seems that there might be a link to injury, if the differences are relatively extreme.

My conclusion is, technical and tactical aspects are more important compared to the physical aspect. Furthermore, I also believe that training both legs/feet of the players also represent more
modern football (especially in youth development), which might also in return lower preferred to non-preferred kicking leg asymmetries/dysbalances.